In recent years, in response to advancements in high-density packaging such as power transistors and hybrid ICs, metal base circuit substrates (such as metal-based wiring substrates and metal-based substrates) have been proposed that take due consideration of the heat generated from these heat-generating components. For example, as disclosed in Patent Documents 1 and 2, technology has been reported in which, in order to enhance heat dissipation from a metal base circuit substrate and stabilize operation of the mounted components, a heat-conductive filler (such as an inorganic powder or a spherical inorganic filler) is included within an insulating film (such as an adhesive layer) that is interposed between the metal substrate (also referred to as the “metal base”) and the conductive foil used for forming the circuit (such as a thin metal or metal thin layer). In Patent Documents 1 and 2, an insulating film composed of a polymeric compound such as an epoxy resin is used as the insulating film, and alumina, aluminum nitride, boron nitride or some other compound is used as the heat-conductive filler.
On the other hand, it is known that liquid crystalline polyesters exhibit high thermal conductivity compared with epoxy resins and the like, and use of such liquid crystalline polyesters as the material for the insulating film that constitutes part of a metal base circuit substrate can be considered.